An Auckland primary school has been evacuated and a man has been decontaminated after two pool chemicals reacted badly when mixed together today.

Fire Service spokeswoman Tania Matthews said they were called to Hillpark Primary School, in Manurewa, at 10.17am by St John ambulance staff who treated a man who was "overcome" when he breathed in the fumes.

Matthews said the man was decontaminated at the school in Grande Vue Rd.

She said the school had been evacuated and fans were being used to clear the fumes.

It was not known when the children would be allowed to return to the school, but Matthews said shortly after midday that firefighters would remain at the scene for some time.